Bandpass filter

ABSTRACT

A bandpass filter includes a dielectric substrate, a resonator electrode that is provided on a portion of a plane at an intermediate height in the thickness direction of the dielectric substrate so as to oppose the top face of the dielectric substrate and includes an aperture, first and second ground electrodes provided over and under the resonator electrode, respectively, so as to oppose the resonator electrode with dielectric layers disposed therebetween and so as to sandwich the resonator electrode, input-output coupling electrodes coupled to the resonator electrode, input-output terminal electrodes that are provided on the outside surface of the dielectric substrate and are electrically connected to the input-output coupling electrodes, and a via-hole electrode that penetrates through the aperture in the thickness direction of the dielectric substrate so as not to be electrically connected to the resonator electrode and is electrically connected to the first and second ground electrodes.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to bandpass filters used in, forexample, communication equipment for a band from microwaves tomillimeter waves and, more particularly, to a bandpass filter configuredto suppress spurious signals that occur depending on the positionalrelationship between ground electrodes and a resonator electrode.

[0003] 2. Description of the Related Art

[0004] Various dual-mode bandpass filters have been used as bandpassfilters for use in high-frequency bands.

[0005] For example, a dual-mode bandpass filter including a resonatorelectrode having an aperture is disclosed in Japanese Unexamined PatentApplication Publication No. 2001-237610. FIG. 17A is a cross-sectionalfront view and FIG. 17B is a plan view, schematically showing adual-mode bandpass filter 101 disclosed in this publication. Thedual-mode bandpass filter 101 includes a dielectric substrate 102. Aresonator electrode 103 is provided at an intermediate height in thedielectric substrate 102. The resonator electrode 103 includes anaperture 103 a (an area where no electrode is formed in the resonatorelectrode 103). The resonator electrode 103 includes a plurality ofnon-degenerate resonant modes. The aperture 103 a is arranged to couplethe resonant modes to each other to define the dual-mode bandpassfilter. Ground electrodes 104 and 105 are provided beneath the topsurface and on the bottom surface of the dielectric substrate 102,respectively, so as to oppose the resonator electrode 103. Referring toFIG. 17B, input-output coupling electrodes 106 and 107 are connected tothe resonator electrode 103. The input-output coupling electrodes 106and 107 extend outside the resonator electrode 103, although not shownin FIG. 17A, and are electrically connected to the correspondinginput-output terminal electrodes (not shown).

[0006] Usually, in a bandpass filter, such as the dual-mode bandpassfilter 101, including the ground electrodes provided over and under theresonator electrode with dielectric layers of the dielectric substratedisposed therebetween, ground electrodes are also provided on sidesurfaces of the dielectric substrate 102. Accordingly, the groundelectrodes define a waveguide, that is, the resonator electrode 103 isprovided in a waveguide. With such a structure, resonances occurdepending only on the shape of the waveguide. Consequently, thestructure, similar to a waveguide, including the ground electrodes islarger than the resonator electrode 103.

[0007] The fundamental resonances caused by the ground electrodes occurat frequencies lower than the resonant frequency of the resonatorelectrode 103, and higher-mode resonances sequentially occur atoverlapping positions with resonant modes caused by the resonatorelectrode 103. Such resonances caused by the ground electrodes produceundesired spurious signals in the dual-mode bandpass filter 101, suchthat it is impossible to achieve good transmission characteristics.

SUMMARY OF THE INVENTION

[0008] To overcome the problems described above, preferred embodimentsof the present invention provide a bandpass filter that preventsundesired spurious signals caused by the resonances of the groundelectrodes to achieve good transmission characteristics.

[0009] According to a preferred embodiment of the present invention, abandpass filter includes a dielectric substrate, a resonator electrode,first and second ground electrodes, input-output coupling electrodes,input-output terminal electrodes, and a via-hole electrode. Theresonator electrode is provided on a portion of a plane at anintermediate height in the thickness direction of the dielectricsubstrate so as to be opposed to the top surface of the dielectricsubstrate and includes an aperture. The first and second groundelectrodes are arranged over and under the resonator electrode,respectively, in the thickness direction of the dielectric substrate soas to oppose the resonator electrode with dielectric layers disposedtherebetween and so as to sandwich the resonator electrode. Theinput-output coupling electrodes are coupled to the resonator electrode.The input-output terminal electrodes are provided on the outside surfaceof the dielectric substrate and are electrically connected to theinput-output coupling electrodes. The via-hole electrode penetratesthrough the aperture in the thickness direction of the dielectricsubstrate so as not to be electrically connected to the resonatorelectrode and is electrically connected to the first and second groundelectrodes.

[0010] The bandpass filter preferably includes second via-holeelectrodes that are arranged in an area outside of the resonatorelectrode in plan view of the resonator electrode and that areelectrically connected to the first and second ground electrodes.

[0011] It is preferable that the resonator electrode be configured so asto have a plurality of non-degenerate resonant modes, and such that theplurality of resonant modes are coupled to each other by the aperture todefine the dual-mode bandpass filter.

[0012] The resonator electrode is preferably a ring resonator electrode.In such a case, controlling the coupling points to the input-outputcoupling electrodes provides the dual-mode bandpass filter.

[0013] The bandpass filter according to a preferred embodiment of thepresent invention is configured such that at least first and secondground electrodes are arranged over and under the resonator electrode soas to sandwich the resonator electrode. The bandpass filter includes thevia-hole electrode that penetrates through the aperture in the resonatorelectrode and is electrically connected to the first and second groundelectrodes. The via-hole electrode shifts the frequency of undesiredspurious signals caused by the resonances of the ground electrodes toachieve good transmission characteristics that are not affected by thespurious signals.

[0014] The second via-hole electrodes in an area outside the resonatorelectrode cause the undesired spurious signals produced by theresonances of the ground electrodes to be spaced further away from thepassband of the bandpass filter to achieve better transmissioncharacteristics. The formation of the second via-hole electrodesprevents the variation in the frequency of the spurious signals evenwhen a variation in the chip size is caused by the manufacturing errorsof the bandpass filter. Hence, the bandpass filter has less variation incharacteristics caused by the manufacturing errors.

[0015] When the resonator electrode is configured so as to have the aplurality of non-degenerate resonant modes and such that the resonantmodes are coupled to each other by the aperture to define the dual-modebandpass filter, the bandpass filter does not have any restrictions onthe coupling points to the resonator electrode and provides various bandcharacteristics by selecting the shapes of the resonator electrode andthe aperture.

[0016] Other features, elements, characteristics, steps and advantagesof the present invention will become more apparent from the followingdetailed description of preferred embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1A is a perspective view of a dual-mode bandpass filteraccording to a first preferred embodiment of the present invention;

[0018]FIG. 1B is a bottom view of the dual-mode bandpass filter of thefirst preferred embodiment of the present invention;

[0019]FIG. 1C is a cross-sectional side view of the dual-mode bandpassfilter of the first preferred embodiment taken along line X1-X1 in FIG.1B;

[0020]FIG. 2 is a plan view schematically showing a resonator electrodeand input-output coupling electrodes of the dual-mode bandpass filter ofthe first preferred embodiment of the present invention;

[0021]FIG. 3 shows S-parameter frequency characteristics in a structurewhere a first via-hole electrode and the resonator electrode are removedfrom the dual-mode bandpass filter of the first preferred embodiment ofthe present invention;

[0022]FIG. 4 shows S-parameter frequency characteristics in a structurewhere the resonator electrode are removed from the dual-mode bandpassfilter of the first preferred embodiment of the present invention;

[0023]FIG. 5 shows S-parameter frequency characteristics of thedual-mode bandpass filter of the first preferred embodiment of thepresent invention;

[0024]FIG. 6 is a plan view schematically showing a known dual-modebandpass filter for comparison;

[0025]FIG. 7 shows S-parameter frequency characteristics of the knowndual-mode bandpass filter shown in FIG. 6;

[0026]FIG. 8A is a plan view and FIG. 8B is a cross-sectional front viewschematically showing the electric field distribution of a knowndual-mode bandpass filter having no via-hole electrode;

[0027]FIG. 9A is a plan view and FIG. 9B is a cross-sectional front viewschematically showing the electric field distribution of the dual-modebandpass filter of the first preferred embodiment of the presentinvention;

[0028]FIG. 10A is a bottom view of a dual-mode bandpass filter accordingto a second preferred embodiment of the present invention;

[0029]FIG. 10B is a cross-sectional side view of the dual-mode bandpassfilter of the second preferred embodiment taken along line X2-X2 in FIG.10A;

[0030]FIG. 11 shows S-parameter frequency characteristics of thedual-mode bandpass filter of the second preferred embodiment of thepresent invention;

[0031]FIG. 12 includes schematic plan views illustrating a case in whichthe width of the dual-mode bandpass filter is decreased;

[0032]FIG. 13 shows S-parameter frequency characteristics of thedual-mode bandpass filter of the first preferred embodiment when theresonator electrode is removed and when the width is decreased;

[0033]FIG. 14 shows S-parameter frequency characteristics of thedual-mode bandpass filter of the second preferred embodiment whenresonator electrode is removed and when the width is decreased;

[0034]FIG. 15 is a cross-sectional front view showing a modification ofthe dual-mode bandpass filter of the present invention;

[0035]FIG. 16 is a plan view schematically showing another modificationof the dual-mode bandpass filter of the present invention; and

[0036]FIG. 17A is a cross-sectional front view and FIG. 17B is a planview, schematically showing a known dual-mode bandpass filter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0037] Preferred embodiments of the present invention will now bedescribed with reference to the drawings.

[0038]FIG. 1A is a perspective view of a dual-mode bandpass filter 1according to a first preferred embodiment of the present invention. FIG.1B is a bottom view of the dual-mode bandpass filter. FIG. 1C is across-sectional side view of the dual-mode bandpass filter taken alongline X1-X1 in FIG. 1B.

[0039] The dual-mode bandpass filter 1 preferably includes asubstantially rectangular dielectric substrate 2. The dielectricsubstrate 2 is preferably made of a suitable dielectric material. Suchdielectric material includes a synthetic resin such as fluoroplastic anddielectric ceramics.

[0040] The dielectric substrate 2 includes a resonator electrode 3 andinput-output coupling electrodes 4 and 5 disposed at an intermediateheight. According to the first preferred embodiment, the dielectricsubstrate 2 includes a plurality of dielectric layers. The resonatorelectrode 3 is disposed on a dielectric layer other than the top layer.FIG. 2 is a plan view schematically showing the planar shape of theresonator electrode 3 and the input-output coupling electrodes 4 and 5.The resonator electrode 3, which preferably has a substantiallyrectangular shape, includes an aperture 3 a at its central portionthereof. The resonator electrode 3 includes a metal film whosecomposition is not particularly limited, such as a metal film made ofaluminum, copper, or other suitable material or an alloy film. Theinput-output coupling electrodes 4 and 5 are also made of a similarmetallic material.

[0041] The resonator electrode 3 being such a metal film is provided ona portion of a plane at an intermediate height in the dielectricsubstrate 2.

[0042] The input-output coupling electrodes 4 and 5 may be arranged atany appropriate positions as long as they are capable of being coupledto the resonator electrode 3. That is, the input-output couplingelectrodes 4 and 5 may be disposed at a height that is different fromthe height at which the resonator electrode 3 is disposed.

[0043] The resonator electrode 3 has a shape so as to generate aplurality of non-degenerate resonant modes. Since the resonatorelectrode 3 includes the aperture 3 a, the plurality of resonant modesare coupled to each other to provide the bandpass filtercharacteristics, as disclosed in the publication described above.

[0044] In the dual-mode bandpass filter 1, a first ground electrode 6 isprovided on an upper level of the dielectric substrate 2 so as to opposethe resonator electrode 3 with dielectric layers therebetween. Althoughthe first ground electrode 6 is disposed inside the dielectric substrate2, the first ground electrode may be disposed on the top surface of thedielectric substrate 2.

[0045] A second ground electrode 7 is disposed beneath the bottomsurface of the dielectric substrate 2 so as to oppose the resonatorelectrode 3 with dielectric layers therebetween. It is not necessary todispose the second ground electrode 7 beneath the dielectric substrate2. The second ground electrode 7 may be embedded at a height above thebottom surface of the dielectric substrate 2.

[0046] The first ground electrode 6 and the second ground electrode 7are preferably larger than the resonator electrode 3, and the resonatorelectrode 3 is sandwiched between the first ground electrode 6 and thesecond ground electrode 7.

[0047] As shown in FIG. 1A, third ground electrodes 8 are provided onopposing sides of the dielectric substrate 2. The third groundelectrodes 8 are electrically connected to the first ground electrode 6and the second ground electrode 7.

[0048] As shown in FIG. 1C, a first via-hole electrode 9 is provided soas to penetrate through the aperture 3 a in the resonator electrode 3.The first via-hole electrode 9 is electrically connected to the firstground electrode 6 and the second ground electrode 7.

[0049] The input-output coupling electrode 4 is electrically connectedto an input-output terminal electrode 10 through a third via-holeelectrode 12, and the input-output coupling electrode 5 is electricallyconnected to an input-output terminal electrode 11 through a thirdvia-hole electrode 13. The input-output terminal electrodes 10 and 11are provided beneath the bottom face of the dielectric substrate 2.

[0050] The operation and effect of the dual-mode bandpass filter 1according to the first preferred embodiment will now be described.

[0051] When input signals are supplied from one of the input-outputterminal electrodes 10 and 11 to the dual-mode bandpass filter 1according to the first preferred embodiment, which includes theresonator electrode 3 and the aperture 3 a that are provided asdescribed above, a plurality of non-degenerate resonant modes occur inthe resonator electrode 3. The resonant modes are coupled to each otherby the aperture 3 a, such that the other electrode of the input-outputterminal electrodes 10 and 11 yields bandpass filter characteristics.

[0052] As described above, the resonator electrode 3 is surrounded bythe first ground electrode 6, the second ground electrode 7, and thethird ground electrodes 8 in a known dual-mode bandpass filter of thistype. More specifically, the first ground electrode 6, the second groundelectrode 7, and the third ground electrodes 8 define a waveguide, and,therefore, the resonance in the waveguide is apt to be spurious.

[0053] In contrast, with the dual-mode bandpass filter 1 according tothe first preferred embodiment, the formation of the first via-holeelectrode 9 suppresses undesirable spurious signals caused by theresonances of the first ground electrode 6, the second ground electrode7, and the third ground electrodes 8. This will be described below withreference to FIGS. 3 to 7 based on specific experiments.

[0054] In the experiments below, the size of the dielectric substrate 2used, which is made of ceramic including magnesium and silicon asprimary ingredients, is about 2.5 mm wide by about 3.2 mm long by about1.0 mm thick. The resonator electrode 3 has a size of about 1.4 mm wideby about 1.5 mm long, and the aperture 3 a has an area of about 0.54mm².

[0055]FIG. 3 shows S-parameter frequency characteristics in aconfiguration in which the resonator electrode 3 and the first via-holeelectrode 9 are removed from the dual-mode bandpass filter 1 accordingto the first preferred embodiment. FIG. 3 shows that resonance indicatedby an arrow A occurs at a frequency of about 26.46 GHz for acharacteristic S11. This resonance corresponds to the resonanceresulting from the configuration having the first ground electrode 6,the second ground electrode 7, and the third ground electrodes 8. Morespecifically, the fundamental resonance caused by the first to thirdground electrodes 6 to 8 occurs at about 26.46 GHz.

[0056]FIG. 4 shows S-parameter frequency characteristics when only theresonator electrode 3 is removed from the dual-mode bandpass filter 1according to the first preferred embodiment. That is, the structure inFIG. 4 is the same as the structure having the transmissioncharacteristics shown in FIG. 3 except for the provision of the firstvia-hole electrode 9.

[0057] As shown by arrow Aa in FIG. 4, the provision of the firstvia-hole electrode 9 causes the fundamental resonance caused by thefirst to third ground electrodes 6 to 8 to occur at a frequency of about31.32 GHz.

[0058] The comparison between FIG. 3 and FIG. 4 shows that the provisionof the first via-hole electrode 9 increases the frequency of thefundamental resonance caused by the first to third ground electrodes 6to 8 by about 5 GHz.

[0059] Thus, the first via-hole electrode 9 shifts the frequency of thefundamental resonance and the frequency of a higher-mode resonancecaused by the first to third ground electrodes 6 to 8 toward higherfrequencies.

[0060]FIG. 5 shows S-parameter frequency characteristics of thestructure shown in FIGS. 1A to 1C. Referring to FIG. 5, the resonantmodes around a frequency of about 25.5 GHz represent the resonant modesproduced by the resonator electrode 3. The resonant modes are coupled toeach other by the formation of the aperture 3 a, thus achieving thebandpass filter characteristic. In contrast, the resonance caused by thefirst to third ground electrodes 6 to 8 occurs at a frequency of about30.73 GHz, as shown by an arrow Ab. Accordingly, as shown in FIG. 5, theresonant frequency of the resonator electrode 3 for providing thebandpass filter characteristics is different from the resonant frequencyof the first to third ground electrodes 6 to 8.

[0061] For comparison, the transmission characteristics of a knowndual-mode bandpass filter 121 shown in FIG. 6 were measured. Thedual-mode bandpass filter 121 has the same structure as the dual-modebandpass filter 1 in FIGS. 1A to 1C except for the removal of the firstvia-hole electrode 9.

[0062]FIG. 7 shows S-parameter frequency characteristics of thedual-mode bandpass filter 121. Referring to FIG. 7, the resonancescaused by the resonator electrode 3 occur around a frequency of about27.7 GHz in the known dual-mode bandpass filter 121. The resonancescaused by the ground electrodes occur at about 25.58 GHz and about 32.49GHz, as shown by arrows Ac and Ad. That is, the fundamental resonancesand the higher-mode resonances caused by the ground electrodes occur onboth sides of the passband of the dual-mode bandpass filter 121.

[0063] The comparison between FIG. 5 and FIG. 7 shows that, in thedual-mode bandpass filter 1 according to the first preferred embodiment,the first via-hole electrode 9 eliminates the effect of undesirablespurious signals caused by the resonances of the first to third groundelectrodes 6 to 8, thus achieving good transmission characteristics.

[0064] The first via-hole electrode 9 is configured such that theresonance caused by the first to third ground electrodes 6 to 8 arrangedso as to surround the resonator electrode occurs outside the passband ofthe dual-mode bandpass filter, as described above. This formationeliminates the effect of undesirable spurious signals caused by theresonances of the first to third ground electrodes 6 to 8, thusachieving good transmission characteristics, as in the first preferredembodiment.

[0065] Since the first to third ground electrodes 6 to 8 are provided soas to surround the resonator electrode 3 in the dual-mode bandpassfilter 1 according to the first preferred embodiment, the radiation fromthe resonator electrode 3 is suppressed so as suppress an increase inthe insertion loss of the filter caused by radiation loss and to preventthe dual-mode bandpass filter from acting as a noise source. A shift infilter characteristics, which occurs when other electronic parts, acasing, or other components are disposed close to the dual-mode bandpassfilter 1 is also suppressed.

[0066] The reason that the spurious signals caused by the resonances ofthe first to third ground electrodes 6 to 8 are shifted by providing thefirst via-hole electrode 9 will be described below.

[0067]FIG. 8A is a plan view and FIG. 8B is a cross-sectional front viewschematically showing the electric field distribution at the fundamentalresonant frequency of the first to third ground electrodes 6 to 8, thatis, the electric field distribution at about 26.46 GHz, in the knowndual-mode bandpass filter. In this electric field distribution, theelectric field strengthens as the density of black stripes increases. Asshown in FIGS. 8A and 8B, a strong electric field occurs at the centralportion on the main surface of the dielectric substrate.

[0068] In contrast, FIG. 9A is a plan view and FIG. 9B is across-sectional front view schematically showing the electric fielddistribution at the fundamental resonant frequency of the first to thirdground electrodes 6 to 8, that is, the electric field distribution atabout 31.32 GHz, in the dual-mode bandpass filter 1 according to thefirst preferred embodiment having the first via-hole electrode 9. Asshown in FIGS. 9A and 9B, the first via-hole electrode 9 at the centralportion of the main surface of the dielectric substrate 2 eliminates thestrong electric field in FIG. 8.

[0069] In other words, since the first via-hole electrode 9 isshort-circuited to the first ground electrode 6 and the second groundelectrode 7, the electric field does not occur in and around an areawhere the first via-hole electrode 9 is provided. Hence, according tothe first preferred embodiment, the first via-hole electrode 9 preventsthe occurrence of a strong resonance at the central portion of thedielectric substrate 2, or prevents the periphery of the first via-holeelectrode 9 from contributing to the resonance caused by the first tothird ground electrodes 6 to 8. As a result, the structure defining thewaveguide is reduced in size so as to increase the frequency of thefundamental resonance caused by the first to third ground electrodes 6to 8.

[0070]FIG. 10A is a bottom view of a dual-mode bandpass filter 21according to a second preferred embodiment of the present invention.FIG. 10B is a cross-sectional side view of the dual-mode bandpass filter21 taken along line X2-X2 in FIG. 10A.

[0071] The dual-mode bandpass filter 21 of the second preferredembodiment is configured in the same manner as the dual-mode bandpassfilter 1 of the first preferred embodiment except that second via-holeelectrodes 22 to 25 are provided. In the plan view of the dual-modebandpass filter 21, a plurality of second via-hole electrodes 22 to 25are provided outside an area where the resonator electrode 3 isprovided. The second via-hole electrodes 22 to 25 are electricallyconnected to the first ground electrode 6 and the second groundelectrode 7, like the first via-hole electrode 9.

[0072] In the dual-mode bandpass filter 21, the second via-holeelectrodes 22 to 25 shifts undesired spurious signals caused by theresonances of the first to third ground electrodes 6 to 8 toward higherfrequencies to reduce the effect of the spurious signals. This will bedescribed below with reference to FIGS. 11 to 13.

[0073]FIG. 11 shows S-parameter frequency characteristics of thedual-mode bandpass filter 21. The fundamental resonance caused by thefirst to third ground electrodes 6 to 8 occurs at about 30.73 GHz inFIG. 5, showing the transmission characteristics of the dual-modebandpass filter 1 according to the first preferred embodiment, while thefundamental resonance caused by the first to third ground electrodes 6to 8 occurs at a higher frequency of about 33.56 GHz in FIG. 11.Referring to FIG. 11, the resonances caused by the resonator electrode 3occur around 25.5 GHz.

[0074] In the dual-mode bandpass filter 21 according to the secondpreferred embodiment, the addition of the second via-hole electrodes 22to 25 shifts undesired spurious signals caused by the resonances of thefirst to third ground electrodes 6 to 8 toward higher frequencies tofurther reduce the effect of the spurious signals. This is because thesecond via-hole electrodes 22 to 25 produce an area that does notcontribute to the resonance around the second via-hole electrodes 22 to25, thus reducing the size of the structure defining the waveguide ascompared with the dual-mode bandpass filter 1 of the first preferredembodiment, and increasing the resonant frequency of the first to thirdground electrodes 6 to 8.

[0075] With the dual-mode bandpass filter 21, the frequency variationsdue to manufacturing errors are reduced. It is assumed that the width Wof the dual-mode bandpass filter is decreased to W1 due to themanufacturing errors, as shown in the diagram at the right in FIG. 12.

[0076]FIG. 13 shows S-parameter frequency characteristics of thedual-mode bandpass filter 1 when the resonator electrode 3 is removedand when the width is decreased as described above. FIG. 14 showsS-parameter frequency characteristics of the dual-mode bandpass filter21 when the resonator electrode 3 is removed and when the width isdecreased as described above.

[0077] The comparison between FIG. 13 and FIG. 4 shows that, when thewidth is decreased due to the manufacturing errors in the dual-modebandpass filter 1, the fundamental resonant frequency of the spurioussignals caused by the resonances of the first to third ground electrodes6 to 8 shifts from about 31.32 GHz to about 32.87 GHz.

[0078] The comparison between FIG. 11 and FIG. 14 shows that, when thewidth is decreased in the dual-mode bandpass filter 21 of the secondpreferred embodiment, the fundamental resonant frequency of the spurioussignals caused by the resonances of the first to third ground electrodes6 to 8 shifts from about 33.56 GHz to about 33.98 GHz.

[0079] In the dual-mode bandpass filter 21, the shift in the resonantfrequency of the spurious signals when the chip size varies is reducedas compared to the shift in the dual-mode bandpass filter 1. In otherwords, the variation in the frequency of the spurious signals caused bythe variation in the chip size resulting from the manufacturing errorsis reduced in the dual-mode bandpass filter 21, thus reducing thevariation in the transmission characteristics.

[0080] The reasons that the variations in the frequency of the spurioussignals caused by the variation in the chip size are reduced in thedual-mode bandpass filter 21, as described above, will be describedbelow.

[0081] In the dual-mode bandpass filter 1, the variation in widthchanges the size of spaces between the central first via-hole electrode9 and both longitudinal sides of the dual-mode bandpass filter 1. Sincethe resonance in a transverse electric (TE) mode depends on the size ofthe spaces, the frequency varies with the variation in the size of thespaces.

[0082] In contrast, in the dual-mode bandpass filter 21, since thespaces are fixed by the second via-hole electrodes 22 to 25 around theresonator electrode 3 and the central first via-hole electrode 9, anyvariation in the width of the chip does not cause a change in the sizeof the spaces. Hence, the variation in the spurious signals caused bythe manufacturing errors is suppressed in the dual-mode bandpass filter21.

[0083]FIG. 15 shows a modified dual-mode bandpass filter 26. As shown inFIG. 15, the first ground electrode 6 and the second ground electrode 7are provided inside the dielectric substrate 2.

[0084] Although the aperture as disclosed in the publication describedabove causes the plurality of non-degenerate resonant modes to becoupled to each other to provide the bandpass filter characteristics inthe dual-mode bandpass filter 1 of the first preferred embodiment andthe dual-mode bandpass filter 21 of the second preferred embodiment, thepresent invention is not limited to such bandpass filters. For example,the present invention can also be applied to a known dual-mode bandpassfilter in FIG. 16, having a ring resonator electrode 31. With the knowndual-mode bandpass filter, controlling coupling points 32 and 33 to thering resonator electrode 31 provides the bandpass filtercharacteristics. A feedback circuit 34 is provided to control the degreeof coupling in the known dual-mode bandpass filter in FIG. 16.

[0085] As described above, the present invention can be applied tovarious bandpass filters using resonator electrodes with various shapes,as long as the resonator electrodes have the respective apertures.

[0086] The present invention is not limited to the above-describedpreferred embodiments, but can be modified in the scope of the attachedclaims. Further, the technologies disclosed in the above-describedpreferred embodiments can be used in combination, as desired.

What is claimed is:
 1. A bandpass filter comprising: a dielectricsubstrate; a resonator electrode provided on a portion of a plane at anintermediate height in the thickness direction of the dielectricsubstrate so as to oppose a top surface of the dielectric substrate andincludes an aperture; first and second ground electrodes arranged overand under the resonator electrode, respectively, in the thicknessdirection of the dielectric substrate so as to oppose the resonatorelectrode with dielectric layers disposed therebetween and so as tosandwich the resonator electrode; input-output coupling electrodescoupled to the resonator electrode; input-output terminal electrodesprovided on the outside surface of the dielectric substrate and beingelectrically connected to the input-output coupling electrodes; and avia-hole electrode that penetrates through the aperture in the thicknessdirection of the dielectric substrate so as not to be electricallyconnected to the resonator electrode and that is electrically connectedto the first and second ground electrodes.
 2. A bandpass filteraccording to claim 1, further comprising second via-hole electrodesprovided in an area outside the resonator electrode in a plan view ofthe resonator electrode and that are electrically connected to the firstand second ground electrodes.
 3. A bandpass filter according to claim 1,wherein the resonator electrode is arranged to have a plurality ofnon-degenerate resonant modes and such that the plurality of resonantmodes are coupled to each other by the aperture to define a dual-modebandpass filter.
 4. A bandpass filter according to claim 1, wherein theresonator electrode is a ring resonator electrode.
 5. A bandpass filteraccording to claim 1, wherein the first and second ground electrodes aredisposed on an upper and lower surface of the dielectric substrate,respectively.
 6. A bandpass filter according to claim 1, wherein thefirst and second ground electrodes are disposed inside of the dielectricsubstrate, respectively.
 7. A bandpass filter according to claim 1,wherein the resonator electrode has a substantially rectangular shape.8. A bandpass filter according to claim 1, wherein said input-outputcoupling electrodes are disposed on a portion of a plane at anintermediate high in the thickness direction of the dielectricsubstrate.
 9. A bandpass filter according to claim 1, wherein theaperture is disposed at a central portion of the resonator electrode.10. A bandpass filter according to claim 1, wherein the first and secondground electrodes are larger than the resonator electrode.
 11. Abandpass filter according to claim 1, further comprising at least oneadditional via-hole electrode connected to at least one of theinput-output electrodes.